This invention relates to a crystal growth procedure and apparatus for growing single crystals of III-V or II-VI materials. The technique is capable of producing extended length single crystals having a highly uniform chemical composition, a low and uniform dislocation density, and a uniform concentration of dopants throughout the crystal.
There are several different growth techniques for producing single crystals of compound semiconductors to be used as substrates for luminescent diodes, transistors, and other discreet electronic devices as well as substrates for integrated circuit chips. The III-V and the II-VI compounds are formed by the combination of elements from Group III and Group V or from Group II and Group VI, respectively. Crystal growth difficulties for such compounds arise from a combination of high melting temperatures, reactivities of the elements or the compound with boat or ampoule materials at elevated temperatures, large differences between the vapor pressures of the constituents of a compound at elevated temperature, large differences between the thermal conductivity of the melt and that of the solid, and in some cases, a high vapor pressure at the melting temperature. Even when single crystals are grown, crystal quality can be adversely affected by having an impurity content, a dislocation density, or an excess of one of the constituents which is too high and/or non-uniform in distribution. Crystals suffering from these defects result in electronic devices which have neither optimum nor reproducible operating parameters, are difficult to process reproducibly, and have a low reliability for integrated circuit applications. All the previously known growth techniques produce crystals that have one or more of the deficiencies listed above.
One known crystal growth technique is the Bridgman technique. Basically a boat of molten GaAs, for example, sits in a uniform temperature zone above the melting point of GaAs, except for one end which contains a single crystal seed and protrudes into a temperature zone below the melting point of GaAs. Crystal growth is accomplished by moving the boat relative to the furnace such that the end of the boat containing the single crystal seed travels out of the melt zone into the cooler zone. Typically the boat containing the GaAs and the surrounding ampoule, which contains an arsenic atmosphere, are made of silica. There are generally two problems with Bridgman crystals. The impurity concentration is too high due to a reaction between Ga or GaAs and SiO.sub.2 at elevated temperatures, and the concentration of certain impurities or dopants is non-uniform across the length of the solidified boule.
Another growth technique is the gradient freeze procedure. An example of a gradient freeze technique is disclosed in Gault, U.S. Pat. No. 4,521,272. This technique differs from the Bridgman technique in that the boat of molten GaAs sits in a temperature gradient rather than in a uniform temperature zone. The temperature gradient is such that most of the single crystal seed is at a temperature below the melting point of GaAs, and the rest of the boat is at a temperature above the melting point of GaAs. The location of the melting temperature of GaAs is such that part of the starting single crystal seed gets melted. Crystal growth is accomplished by slowly lowering the temperature of the boat while maintaining the temperature gradient across the boule of the boat. The boat or crucible is usually made of either SiO.sub.2 or pyrolytic boron nitride. The problems with this technique are a nonuniform dopant, a high impurity concentration, and a limited length for the resulting crystal. The limitation on crystal length is due to the melting temperature and the temperature gradient that the melt is exposed to. Since GaAs melts at approximately 1236.degree. C. and the rest of the melt is progressively hotter, a very long boat would experience very high temperatures which approach the softening temperature of the SiO.sub.2 ampoule containing the boat and arsenic atmosphere.
A third growth technique is the liquid encapsulation Czochralski (LEC) process. In the LEC process, molten GaAs contained in a pyrolytic boron nitride crucible is encapsulated or covered by a layer of molten B.sub.2 O.sub.3. A single crystal seed is lowered through the molten B.sub.2 O.sub.3 into the molten GaAs. If the melt temperature and the temperature gradient across the GaAs/B.sub.2 O.sub.3 interface is optimal, then crystal growth is accomplished by slowly withdrawing the seed and slowly lowering the melt temperature. Problems encountered in LEC crystals are that the dislocation density is too high and is non-uniform across the diameter of the crystal, the impurity or dopant concentration is non-uniform across the length of the crystal, and the concentration of constituents is non-uniform across the diameter and across the length of the crystal.
Other attempts have been made to grow single crystals of III-V or II-VI material. All have failed to produce crystals of optimal quality. Swiggard, "Liquid Encapsulation Zone Refining (LEZOR)", Journal of the Electrochemical Society, Vol. 114, No. 9, pages 976-7 (1967), discloses an early attempt by the present inventor at vertical zone refining with a B.sub.2 O.sub.3 encapsulant. Material to be crystallized and B.sub.2 O.sub.3 were loaded into a quartz capsule and subjected to repeated zone melting, but the resulting crystal always fractured upon removal from the boric oxide.
Johnson, "Liquid Encapsulated Floating Zone Melting of GaAs", Journal of Crystal Growth, Vol. 30 pages 249-56 (1975), attempted vertical zone refining with a B.sub.2 O.sub.3 encapsulant using fused silica glassware. The boric oxide was first melted and a feed rod of GaAs was then immersed therein. A floating molten zone held in place by surface tension was established in a central section of the rod, but the ends of the feed rod were never melted. Polycrystalline GaAs crystals up to 10 mm in diameter were grown, but single crystals could not be obtained. Attempts to work with larger diameter zones resulted in molten zones which sagged badly.
Swiggard et al., "Preparation of Bulk Alloys of III-V Compounds", Inst. Phys. Conf. Ser., No. 65, Ch. 1 (1983), describes horizontal zone refining and crystal growth using GaAs and B.sub.2 O.sub.3 in a pyrolitic boron nitride crucible. After the desired number of molten zone passes, methanol was bubbled through the molten boric oxide to separate it from the grown crystal material. Difficulties were encountered in controlling the shape cf the melt (freezing) interface with this horizontal technique and only polycrystals were obtained.
There is a need for a process and apparatus for growing larger single crystals with more uniform composition.